The present invention relates to a rotary body bearing apparatus and a motor required to operate with extreme rotation accuracy, such as a polygon mirror motor for use in a laser beam printer and, more particular, to a rotary body bearing apparatus having a bearing structure capable of high accuracy in rotation and a motor, such as a polygon mirror motor, having an extremely small rotation fluctuation.
Recently, in a laser beam printer, a magnetic disc apparatus and a video tape recorder, accompanying the high performance and the high function of a motor for used in these apparatuses it requires the high rotation accuracy for the motor.
In a conventional technique, a ball bearing structure is used in this kind of motor, however, for the purpose of providing a high speed characteristic and a low rotation number fluctuation characteristic, a fluid lubrication bearing structure has been adopted in the motor.
Further, from the point of view of maintaining a clean environment, an air bearing structure and a magnetic fluid bearing structure having a superior seal characteristic have been proposed and, at the present time, the air bearing structure and the magnetic fluid bearing structure have been put into practical use.
In particular, in comparison with the air bearing structure, since the magnetic fluid in a magnetic fluid bearing structure has a high viscosity, it is possible to obtain a high bearing rigidity characteristic with a bearing structure having a small inner diameter dimension and, further, to obtain a good attenuation characteristic with respect to the shaft vibration.
As an acceptable bearing apparatus for the high accuracy rotation, such a technique is disclosed in, for example, Japanese patent laid-open No. 512,514/1991.
According to the above stated prior technique, it has been proposed to use a fluid bearing structure having a high speed characteristic and a high accuracy rotation characteristic for a motor, and further, in order to be able to use the motor in a clean environment the magnetic fluid is employed in the lubrication and sealing of the bearing structure.
However, although a motor using the above disclosed bearing apparatus provides a high accuracy of rotation, there is a limit to which the rotation number fluctuation can be reduced. As a result, there is a case in which such a motor can not be used in the recent models of laser beam printers having a high precision characteristic.
Namely, in the recent laser beam printers, by the electrical control of a polygon mirror motor which is used for effecting uniform speed scanning for the laser light, the rotation speed can be made constant.
The prior laser beam printer has a rotation number fluctuation of 10/1000%-20/1000%, however, in the laser beam printer providing a high precision characteristic and color characteristic, it is necessary to provide a high precision rotation accuracy with a rotation number fluctuation of less than 5/1000%.
In order to realize the above stated high rotation accuracy for the motor it is necessary to lessen the friction fluctuation of the bearing apparatus.
In a fluid bearing apparatus principally using lubricating oil and in which the viscosity of the fluid is constant, the bearing apparatus has no friction fluctuation, however, in the above stated prior bearing apparatus, it was found that it is difficult to realize a rotation number fluctuation which is less than 5/1000%.
Namely, in a high speed rotation region where the speed is more than 10,000 rpm, as a result of an analysis resulting from experimentation by the inventors of the present invention, it was found that the higher the speed of rotation becomes, the larger the rotation number fluctuation becomes. In particular, at a high speed of rotation of more than 20,000 rpm, there is a case in which the rotation number changes irregularly.
So as to more clearly understand the above phenomena and so as to solve the above problems, using a transparent bearing case, the flow of the lubricating oil in a bearing chamber was observed by the inventors of the present invention. As a result, it was found that the conditions of the surrounding portion of the bearing member and of the portion between both bearing members are varied in proportion to the high speed rotation.
Namely, due to the rotation of the rotative shaft, small air bubbles are generated in the lubricating oil, and as the air bubbles grow comparatively large bubbles are generated. Then, as the air bubbles grow to some large degree, large air bubbles are crushed to form fine bubbles. The above stated phenomena has been observed to occur repeatedly in the bearing chamber.
The above stated friction fluctuation of the fluid bearing is caused by the mixing of the air bubbles, however it is difficult to find an example in which a lubricating oil bearing is used in a polygon mirror motor, and at the same time there is no rotary machinery which requires such high accuracy of rotation.
Consequently, heretofore no consideration has been given to the reduction of the air bubbles and the prevention of the occurrence of the air bubbles being generated in the lubricating oil or the magnetic fluid during high speed rotation.